3.10.6 Argument Prescan

Macro arguments are completely macro-expanded before they are
substituted into a macro body, unless they are stringified or pasted
with other tokens. After substitution, the entire macro body, including
the substituted arguments, is scanned again for macros to be expanded.
The result is that the arguments are scanned twice to expand
macro calls in them.

Most of the time, this has no effect. If the argument contained any
macro calls, they are expanded during the first scan. The result
therefore contains no macro calls, so the second scan does not change
it. If the argument were substituted as given, with no prescan, the
single remaining scan would find the same macro calls and produce the
same results.

You might expect the double scan to change the results when a
self-referential macro is used in an argument of another macro
(see Self-Referential Macros): the self-referential macro would be
expanded once in the first scan, and a second time in the second scan.
However, this is not what happens. The self-references that do not
expand in the first scan are marked so that they will not expand in the
second scan either.

You might wonder, “Why mention the prescan, if it makes no difference?
And why not skip it and make the preprocessor faster?” The answer is
that the prescan does make a difference in three special cases:

Nested calls to a macro.

We say that nested calls to a macro occur when a macro's argument
contains a call to that very macro. For example, if f is a macro
that expects one argument, f (f (1)) is a nested pair of calls to
f. The desired expansion is made by expanding f (1) and
substituting that into the definition of f. The prescan causes
the expected result to happen. Without the prescan, f (1) itself
would be substituted as an argument, and the inner use of f would
appear during the main scan as an indirect self-reference and would not
be expanded.

Macros that call other macros that stringify or concatenate.

If an argument is stringified or concatenated, the prescan does not
occur. If you want to expand a macro, then stringify or
concatenate its expansion, you can do that by causing one macro to call
another macro that does the stringification or concatenation. For
instance, if you have

then AFTERX(BUFSIZE) expands to X_BUFSIZE, and
XAFTERX(BUFSIZE) expands to X_1024. (Not to
X_TABLESIZE. Prescan always does a complete expansion.)

Macros used in arguments, whose expansions contain unshielded commas.

This can cause a macro expanded on the second scan to be called with the
wrong number of arguments. Here is an example:

#define foo a,b
#define bar(x) lose(x)
#define lose(x) (1 + (x))

We would like bar(foo) to turn into (1 + (foo)), which
would then turn into (1 + (a,b)). Instead, bar(foo)
expands into lose(a,b), and you get an error because lose
requires a single argument. In this case, the problem is easily solved
by the same parentheses that ought to be used to prevent misnesting of
arithmetic operations:

#define foo (a,b)
or
#define bar(x) lose((x))

The extra pair of parentheses prevents the comma in foo's
definition from being interpreted as an argument separator.